"To write a sequel" means to create a new story or work that continues the events or characters from a previous book, movie, or other artistic creation. Full definition
When it came time to write the sequel for my first book, I wasn't willing to spend another 8 years struggling to get it done. (kevintjohns.com)
I consider it a tragedy of American literature that she never wrote a sequel! (bookpage.com)
The actor - writer will not be involved with writing the sequel script, but he may return to star, depending on how the screenplay turns out. (movieweb.com)